Electric fuse



M. B. woon `2,068,584

ELECTRIC FUSE Filed March 13, 1934 24 fivef-z oif uw e. afm? Patented Jan. 19, 1937 UNITED STATES PATENT OFFICE ELECTRIC FUSE Application March 13, 1934, Serial No. 715,304

12 Claims.

This invention relates to electric fuses wherein the readily fusible portion of the fusible element or fuse link underlies a window in the fuse casing and wherein an end cap retains the transparent window in place and is crimped or bent about a flange of the insulating casing. The invention particularly relates to electric fuses of the screw plug type.

A plug type of fuse is adapted to be screwthreaded into a base or receptacle and is used largely for the protection of the various circuits of a house. The fuse bases in the house circuits are frequently installed in groups in cellars, closets and other relatively dark localities where the intensity of light is so low that it is difficult to determine the condition of the fusible element of the usual plug fuse, that is, to determine whether or not the fuse has blown. It often happens that when one of the fuse circuits is interrupted, a large number of the fuses of a group have to be tested to locate the blown fuse, since the available light is insufficient to enable the blown fusible element of the group of the usual fuses to be surely located.

An object of the present invention is the provision of an electric fuse of the type above set forth wherein the interior of the fuse casing that underlies the more readily fusible portion of the fusible element is white in color and wherein the fusible element or link at and adjacent the more readily fusible portion thereof is black so that the more readily fusible portion contrasts with and thus stands out prominently against the white background provided by the white interior of the fuse chamber of the casing, whereby the condition of the fusible element can be observed readily even in dull light.

A further object is the provision of a fuse chamber having tapered white walls that converge toward the bottom of the chamber so that the light admitted into the large open end of the chamber can be reflected into the bottom of the chamber to illuminate it and cause it to contrast more prominently with the black non-reflecting fusible element that it underlies.

Considerable pressure may be generated within the fuse chamber when the heavily-overloaded fusible element blows and further object of the invention is the provision of a fuse plug having an improved construction for venting the fuse chamber and relieving the pressure in a manner that is free from lire hazard.

A yet further object is generally to improve the construction of electric fuses.

Fig. 1 is a side elevation of an electric fuse of the plug type embodying the present invention.

Fig. 2 is a plan view of the fuse of Fig. l.

Fig. 3 is a sectional elevation taken along line 3 3 of Fig. 2.

Fig. 4 is a detail showing a portion of the gas venting channels and cap-locking teeth of the head flange of the fuse body.

Fig. 5 is a sectional detail taken along line 5 5 of Fig. 3 and illustrating the manner in which the end cap overlies and interlocks with the cap-locking teeth and venting channels.

Fig. 6 is a plan view of a fusible element or fuse link embodying the present invention.

Fig. 7 is a plan view of a strip of fusible metal from which the fusible element of Fig. 6 is severed.

Fig. 8 is a View similar to Fig. 6 but illustrating a modied form of fusible element.

Fig. 9 is a plan view similar to Fig. 2 but illustrating the appearance of a blown fusible element.

The. plug fuse embodying the present invention comprises the generally cylindrical insulating body I formed of dry process porcelain having the fuse chamber I2 therein open at the top of' the body and closed at the bottom by a transverse wall I II which is integral with the side wall of the body. The exterior cylindrical surface of the body I Il is provided with outstanding spirally disposed ridges Il forming external screw threads on which an externally and internally screwthreaded metal shell IB is screw-threaded, which shell constitutes the outer terminal or contact member of the fuse. The fuse body is provided with a center contact 20 in the form of a tubular rivet having a head 22 which overlies the middle portion of the outer face of the bottom wall I4 and a shank 24 which is extended through a passage in the bottom wall and is suitably secured therein as by upsetting the inner end of the shank. The shank has a recess therein which receives one end of a fusible element or fuse link 26, which end is electrically and mechanically connected to the center contact by a body of solder 28. The. other end of the link extends through a passage 30 in the fuse body to a position behind a portion of the shell I8 and is anchored to said shell by a body of solder 32 which also anchors the shell against rotation on the porcelain body. 'I'he upper or head end of the insulating body Ill is provided with a peripherally outstanding continuous ange 34 the lower face 36 of which is inclined or slopes toward the cylindrical shell-bearing portion of the body and is formed, preferably completely about its circumference, with a plurality of gas-Venting channels 38 and interposed cap-locking teeth 40, both of which are inclined toward the shell-bearing portion of the insulating body. A clear transparent mica disc 42, comprising a window overlies the top of the body I0 and the opening of the fuse chamber I2 and is disposed Within a metal end cap 44. Said end cap has an end wall 46 which overlies the transparent window disc 42, a cylindrical wall 48, preferably knurled or having corrugations 5U, which closely surrounds and encloses the cylindical side wall of the flange 34, and an inturned flange or wall 52 which is parallel with the end wall 46 and confronts the inclined wall 36 of the flange 34, and the teeth and channels 38 thereof. The flange 52 preferably is not in contact with the wall 36 eX- cept at the inner edge of the flange so that there is an annular channel 54 between the flange and the wall. At its inner edge, the flange 52 is forced into the channels 38, as indicated in Figs. 1, 3, and 5, sufficiently to form a series of raised portions or ridges 56 which receive the apices of the teeth 4|] by which the end cap is locked against rotation on the insulating body I0. The venting channels are open under the inner edges of the flange 52 so that excessive gas pressure within the fuse chamber I2 can be vented through said channels and a passage between the outer face of the fuse body and the transparent window 42 and the side wall 48 of the end cap, which passage is formed by the high chamber pressure and the expansion of the end cap, permitted largely by the construction of the end flange 52 and its spaced relation with the inclined end face of the ange 34, which allows the end flange to flex upon abnormal chamber pressures.

The end wall 46 of the end cap has an opening 58 which preferably is the full diameter of and is aligned with the fuse chamber I2 and through which the interior of the chamber is visible.

The fusible. element 26 has an intermediate section 6I) which is disposed immediately under the opening58 and the window 42 and is generally parallel therewith and perpendicular to the end portions that are connected to the fuse terminals, Said intermediate portion 6I) contains a more readily fusible section 62 usually of reduced cross-sectional area as by being formed by the opposed notches 64 in the opposite edges of the fusible element, or otherwise. In Fig. 8 the more readily fusible section 62a is obtained by an aperture 64a in the fuse. The more readily fusible section 62, for the type of fusible element herein illustrated, is located usually in the middle of the element and is ruptured upon current overload, thereby to interrupt the circuit including the fuse. On moderate overloads, especially, the line of fusion of the ends of the fusible element at the more readily fusible section 62 is narrow. On heavier overloads the line of fusion may be broader. It is an object of this invention to provide a marked and relatively great contrast between the fusible element and the background against which it appears when viewed through the window 42 so that the condition of the fusible element, and the presence of a ruptured line, if there is one, can be readily observed even when the fuse is in dull light and the observer is some distance therefrom. The fuse body IIJ is made of a dry process porcelain and is practically pure white and hence the inner wall 66 of the fuse chamber thus is readily visible even when the illumination thereof is poor, as when the fuse is in a poorly illuminated locality. The porcelain surface 66 has an apparent crystalline appearance that enhances its response to illumination. The bottom of the fuse chamber I2 is occupied by a body of cement 68, or the like, as plaster of Paris or other material that is white, which overlies and conceals the center Contact so that the entire surface of the fuse chamber presents a uniformly white appearance. The fuse chamber I2 is tapered inwardly or the wall 66 thereof slopes from the open end toward the bottom of the chamber so that the chamber is smaller in diameter at the bottom than at the top. Thus light admitted through the window 42 is reflected from upper portions of the sloping side wall 66 downwardly to the bottom of the fuse chamber so that the entire inner surface of the chamber is practically equally brilliantly illuminated.

The fusible element and particularly that portion at and also preferably adjacent the more readily fusible portion 62 lthereof is so made as to contrast as much as is practicable with the white background provided by the surface 66, which includes the surface of the cement body 63, against which it outstands. To this end the fusible element is provided with a coat of paint 'Hl which contrasts as much as possible with the white background. A dull black paint is preferred, black so that its contrast with the white background is great, and dull so that it will not reflect any material amount of light and reduce the contrast. A colored coating may perhaps be used in place of black as, for instance, red or yellow, but the contrast is not so great and a red or yellow paint soon turns black due to the heating the fusible element. Preferably the black coating occupies about the entire horizontal length of the fusible element that is immediately under the window 42 and at least the more readily fusible section 62 is coated. The top of the fuse thus presents the appearance of a white disc traversed part way by a black stripe, as in Fig. 2, the black stripe standing out prominently in contrast with the white background even in a poorly illuminated locality. The condition of the fusible element may be instantly observed and a break in the element detected Visually by a line of white 1 I, see Fig. 9, traversing the black stripe when the fuse is in its receptacle at some feet from the observer.

The coating 'I0 need be on only that portion of the fusible element that confronts the window. The remainder of the element and the entire other surface thereof may be uncoated and of its natural finish, which is fairly bright when the fusible element comprises a zinc strip as is customary. may be applied in any suitable manner as by painting it on by brush or paint rolls or by printing it on by lithography. Preferably a long sheet 'i2 of the fusible metal, as wide as the fusible element is long, has a lengthwise-extended band 10a of the black coating applied thereto in the proper location and separate fusible elements 26 are severed from the sheet I claim:

l. An electric fuse comprising an enclosing casing having a fuse link containing chamber provided with a transparent window, said charnber having a wall under said window that is of a highly light reflecting nature, and a fusible link having a portion interposed between said window and said wall and having in said portion a readily fusible part, said portion on opposite sides of said part having a heat-resistant highly light absorbent surface that confronts said window and traverses said light reflecting surface The black contrasting coating I0 g whereby the outline of the dark link is readily visible by contrast upon the light background.

2. An electric fuse comprising an enclosing casing having a transparent window, a fusible element in said casing having a part confronting said window that is black and thereby poorly light reflecting and is adapted to fuse within the length of the black part, said casing having a part that confronts said fusible element part, said casing part being white and more readily light reflecting than said black fusible element part by light entering said casing through said window whereby the outline of said fusible element part appears in strong contrast to the white background provided by said white casing part, said black part being comprised of a surface coating that is resistant to the normal heating of the fusible element.

3. An electric fuse comprising an enclosing casing having a transparent window, a fusible element in said casing having a part that confronts said window, said casing having means located under said fusible element part that provides a white background for said fusible element part when viewed through said window, and said fusible element part having a black heat-resistant coating and thereby presenting a black appearance against said white background, said black fusible element part having an intermediate readily fusible section adapted to fuse and indicate the fused condition by a white line of the white background appearing between the severed black ends of the fusible element.

4. An electric fuse comprising an enclosing casing having a transparent window, a fusible element in said casing having a part that confronts said window, said part being provided with an intermediate readily fusible section, said casing having means located under said fusible element part and readily fusible section that provides a white background for said fusible element part when viewed through said window and when illuminated by light admitted through said window, and said fusible element part on opposite sides of said readily fusible section having a heat resistant surface coating that is relatively unresponsive to said light and does not emit light and presents a black outline of the fusible element against said white background.

5. An electric fuse comprising an enclosing casing having a deep fuse chamber that is open at one end and closed at the other end and has a side wall that slopes inwardly from the open end toward the closed end, a closure for said open end presenting a transparent window which is bigger in cross-section than the cross-section of said chamber at the closed end, said sloping wall and closed end presenting a white surface constituting a background, and a fusible element in said chamber having a part that confronts said window and is between it and said background said part having a heat-resistant coating that presents a dark appearance against said white background when viewed through said window, said dark part having an intermediate relatively fusible section.

6. An electric fuse comprising an enclosing casing having a deep fuse chamber that is open at one end and closed at the other end and has a side wall that slopes inwardly from the open end toward the closed end, a contact member in said lower end, a sealing mass in said closed end over said contact member, a closure for said open end. presenting a transparent window which is bigger in cross-section than the cross-section of said chamber at the closed end, said sloping wall and sealing mass presenting a white surface constituting a background, and a fusible element in said chamber having one end extended through said sealing mass and connected to said contact member and having a part that confronts said window and is between it and said background and has a heat resistant black coating thereon that contrasts sharply with said white background when viewed through said window, said black coated part having an intermediate readily fusible section adapted to melt upon overload and thereby expose a white gap between the severed black ends of the fusible element.

'7. An electric fuse comprising an enclosing casing having deep fuse chamber that is open. at one end and closed at the other end and has a side wall that slopes inwardly from the open end toward the closed end, a closure for said open end presenting a transparent window which is bigger in cross-section than the cross-section of said chamber at the closed end, said sloping wall and closed end presenting a white surface constituting a background, and a fusible element in said chamber having a part that confronts said window and is between it and said background and has a heat resistant black coating thereon that contrasts sharply with said white background when viewed through said window, said coating being relatively weakly reflecting and non-light emitting and said background being strongly reflecting to light admitted through said window, said black coated part having an intermediate readily fusible section adapted to melt upon overload and thereby expose a white gap between the severed black ends of the fusible element.

8. An electric fuse comprising an enclosing casing having a deep fuse chamber that is open at one end and closed at the other end and has a side wall that slopes inwardly from the open end toward the closed end, a closure for said open end presenting a transparent window which is bigger in cross-section than the cross-section of said chamber at the closed end, said sloping wall and closed end presenting a white surface constituting a background, and a fusible element having a part that immediately confronts said window and is between it and said background and has an intermediate more readily fusible section, said section and said fusible element part on both sides of said section being provided with a black coating thereon that is resistant to change in appearance by thel normal heating of the element and contrasts sharply with said white background when viewed through said window, said coating being relatively weakly reiiecting and non-light emitting and said background being strongly reecting to light admitted through said window, whereby the outline of the coated part stands out prominently against said white background.

9. A plug type electric fuse comprising an insulating casing having a fuse chamber open at one end and closed at the other end, a center contact and a screw-threaded shell carried by said casing, said casing at said open end having an outstanding flange provided with an upper smooth radial wall and a lower wall that is inclined toward said shell and is provided with a series of inclined teeth and interdental chamber venting channels, a transparent plate upon and overlying the end of said casing and the open end of said chamber, and an end cap on said casing over said plate having an inwardly directed flange which underlies and engages said sloping lower wall and said teeth and is capable of yielding in response to pressure in said charnber to relieve said plate from engagement with said upper wall.

10. A plug type electric fuse comprising an insulating casing having a fuse chamber open at one end and closed at the other end, a center contact and a screw-threaded shell carried by said casing, said casing at said open end having an outstanding flange provided with an upper smooth radial wall and a lower wall that is in clined toward said shell and is provided with a series of inclined teeth and interdental chamber venting channels, a transparent plate upon and overlying the end of said casing and the open end of said chamber, and an end cap on said cas-- ing over said plate having an inwardly directed flange which underlies and engages said sloping lower wall and said teeth, the inner part only of said end cap flange engaging said sloping end Wall and said end cap flange being otherwise spaced from said wall, whereby said end cap ange is capable of yielding in response to pressure within said fuse chamber and permitting said plate to be raised by said pressure away from said upper wall.

11. A plug type electric fuse comprising an insulating casing having a fuse chamber open at one end and closed at the other end, a center Contact and a screw-threaded shell carried by said casing', said casing at said open end having an outstanding flange provided with a smooth radial upper wall and a lower wall that is inclined toward said shell and is provided with a series of inclined teeth and interdental chamber venting channels, a smooth transparent plate bearing upon said upper wall and overlying the open end of said chamber, and an end cap on said casing over said plate having an inwardly directed yieldable iiange which. underlies and engages said sloping lower wall and said teeth, said channels communicating with the atmosphere under the inner edge of said end cap flange,

12. A plug type electric fuse comprising an insulating casing having a fuse chamber open at one end and closed at the other end, a center Contact and a screw-threaded shell carried by said casing, said casing at said open end having an outstanding flange provided with a lower wall that is inclined toward said shell and is provided with a series of inclined teeth and interdental chamber venting channels, a transparent plate overlying the end of said casing and the open end of said chamber, and an end cap on said casing over said plate having an inwardly directed which underlies and engages said sloping lower wall and said teeth, said end cap flange having indentations in which said teeth are located and by which said end cap is anchored to said casing against rotation thereon.

MORRIS B. WOOD. 

